1. Field of the Invention
This invention relates to a control circuit for a variable bridge T attenuator.
2. Description of the Prior Art
A bridge T fixed value attenuator of the type shown in FIG. 1A is well known. An input signal is applied to the Port 1 terminal; an output signal is provided at the Port 2 terminal. The attenuator of FIG. 1A has a main arm which includes two resistors R1 and R2. The node between resistors R1 and R2 is connected to ground via shunt resistor R3. The upper (bridge) arm includes resistor R4. To achieve a prescribed level of attenuation while maintaining a predetermined input and output impedance R0, resistors R1 and R2 are fixed at a value of R0 while the values of resistors R3 and R4 are varied so that R3*R4=R.sub.0.sup.2.
One method for controlling a prior art bridge T attenuator is to use a dual potentiometer connected as in FIG. 1B where resistances R3 and R4 are each potentiometers. However this use of potentiometers has been found to be inferior, because it does not lend itself to high frequencies. Replacing resistors R3 and R4 with pin diodes creates an electrically variable attenuator which has superior frequency response but now needs an electrical drive provided by a simple potentiometer to maintain the equality R3*R4=R.sub.0.sup.2.
A second approach is to use a combination of a microprocessor (or microcontroller) and a lookup table stored in a memory to control the bridge T attenuator. For various degrees of attenuation, different resistance values are stored in the lookup table. When the user selects a particular level of attenuation, the microprocessor accesses the lookup table, determines the corresponding amount of resistance, and then adjusts it accordingly. Such digital control of attenuation is effective but requires relatively expensive components, i.e. at least a microcontroller and a memory.
Thus there is not available a bridge T attenuator control circuit or method which is satisfactory in terms of functionality by providing a prescribed level of attenuation for all values of attenuation while maintaining all terminal impedances constant and equal to the characteristic impedance of the system and which is also relatively inexpensive and simple.